The scenes aboard ship were of a fantasy cruise gone awry. Passengers who were supposed to be spending their days lounging on deck instead suffered bouts of vomiting and were confined to their cabins. On Sunday, a team from the Centers for Disease Control boarded the vessel to investigate during a U.S. Virgin Islands port call. The 10-day voyage that departed New Jersey last week was forced to return from the Caribbean two days early.

Kim Waite of London told The Associated Press that she thought she was the only person who'd taken ill, only to be wheeled into the ship's infirmary packed with sick passengers.

"I started crying, I couldn't believe it," Waite said. "I was in shock."

Royal Caribbean says the ship will be sanitized and no one will be allowed aboard for a period of more than 24 hours as an extra precaution. It is scheduled to sail again on Friday for a nine-day trip to Puerto Rico, St. Thomas, the Dominican Republic and Haiti.

Noroviruses have been the culprit on the vast majority of the cruise-ship outbreaks monitored by the Centers for Disease Control under its Vessel Sanitation Program. They are highly contagious gastrointestinal illnesses that cause inflammation of the stomach and large intestines. There are a number of unpleasant symptoms.

WebMD says noroviruses "have become notorious for sending hundreds of cruise ship passengers at a time running for their respective bathrooms and for steering entire ships back to port early." It adds:

"Back on dry land, noroviruses also have a big impact on people's health. The CDC estimates that noroviruses are responsible for more than half of all food-borne disease outbreaks each year. And noroviruses are the most common cause of diarrhea in adults and the second most common cause in children."

"Once an infected person gets onboard a ship, the virus can be spread quickly, mainly through hand contact with ship railings, bathroom doors, and especially buffet food, said [Michael] Zimring, [the director of the Center for Wilderness and Travel Medicine at Mercy Center in Baltimore, Md.]

"The industrial-size servings of food on a cruise ship with hundreds of passengers can be particularly worrisome, since once the virus enters the food it can spread rapidly. Food can also get more easily contaminated with the virus if it sits out for several hours, as is often the case with buffet-style meals."

The CDC says there have been 56 gastrointestinal outbreaks on cruise ships in the past five years. That might sound like a lot, but in fact, Nat Geo says, it's not any more than in the 1990s or 2000s.